Question

What volume (mL) of the solvent benzene should be added to 142 mL of the solute CH3CN to generate a solution that is 2.68% by mass CH3CN?

Answer 1

You don't have enough information. You need the density of benzene, the density of CH3CN, and the density of the solution.

Answer 2

Here is the other information..
Molar Mass (g/mol)
CH3CN 41.05
C6H6 78.12

Density (kg/m3):
CH3CN 785.7
C6H6 878.6

Name/Formula:
acetonitrile
CH3CN
benzene
C6H6

Answer 3

I assume this is mass/mass percent.
142 mL CH3CN has what mass? mass=vol x density or mass CH3CN = 142 mL x 0.7857 g/mL = 111.569 grams CH3CN.

What mass solution must we have to equal 2.68%? [111.569 g/mass solution]*100 = 2.68% and solve for g solution. g soln = 299.002 g solution. How much of that is CH3CN and how much benzene.
Total mass = 299.002 g
mass CH3CN = 111.569 g
difference = ??g = mass benzene.

volume benzene = mass benzene/density vol benzene = ?? g benzene/0.8786. I would then round the answer to 3 significant figures.
Post your work if you get stuck. Check my thinking. Check my work.